Despite its wide application, AWJ machining is still far from being optimized and the selection of the process variables is often made without considering the cost of the machining or the quality of the generated surfaces. The present work is addressed to the analysis of the optimization of the AWJ machining, considering the quality of the generated surfaces (in terms of surface finish) as a constraint. An experimental analysis has been performed in order to formalize the relationships between process variables and roughness parameters. Then, the cost optimization problem has been studied, using the identified relationship as a constraint It has been found that, in order to reduce the machining cost, the pressure level should be fixed as low as possible, while the abrasive mass flow rate, the mesh number and the feed rate should be fixed considering the constraint. Moreover, if the feed rate is fixed by the production rate of the system, the identified equations (or their graphical representation, the iso-roughness curves) can be used to select both the abrasive size and the abrasive size and the mass flow rate level that minimize the cost of the machining.